This invention relates generally to systems capable of initiating polymerization. More specifically, the invention relates to initiator systems comprising (1) a complexed initiator and (2) decomplexers. The invention further relates to the use of these systems for initiating polymerization, as well as kits, bonding compositions, and polymerized compositions made therewith, coated substrates prepared therefrom, bonded articles prepared therefrom, and methods of preparing such bonded articles.
Systems for initiating the polymerization of monomers to make adhesive compositions are known in the art. U.S. Pat. Nos. 5,106,928, 5,286,821, and 5,310,835 to Skoultchi et al., for example, describe two-part initiator systems for initiating the polymerization of acrylic monomers. The first part of these two-part systems includes a stable organoborane amine complex and the second part includes an activator. The activator liberates the organoborane compound by removing the amine group, thereby allowing the organoborane compound to initiate the polymerization process. Activators are also sometimes referred to as liberators or decomplexers.
U.S. Pat. No. 5,286,821 to Skoultchi describes that suitable activators for liberating the organoborane compound include aldehydes having the general structure:R—(CHO)xwherein R is an alkyl group having 1 to 10 carbon atoms or an aryl group having 6 to 10 carbon atoms and x is 1 to 2. Examples include benzaldehyde; o-, m-, p-nitrobenzaldehyde; 2,4-dichlorobenzaldehyde; p-tolylaldehyde; and 3-methoxy-4 hydroxybenzaldehyde.
U.S. Pat. Nos. 5,310,835 and 5,106,928 describes that suitable activators for liberating the organoborane compound also include organic acids having the structure:R—COOHwherein R is H, an alkyl group, or an alkenyl group having 1 to 8 carbon atoms. Examples include acrylic acid, methacrylic acid, benzoic acid, and p-methoxybenzoic acid.
Fujisawa, Imai, and Mashuhara also describe a system for initiating the polymerization of methyl methacrylate (See, Reports of the Institute for Medical and Dental Engineering, vol. 3, page 64 (1969)). The system comprises a trialkylborane amine complex and an activator such as the chloride of methacrylic or n-butane sulfonic acid, terephthalic acid chloride, benzoyl chloride, p-toluene sulfonic acid chloride, benzene sulfonic acid chloride, methane sulfonic acid chloride, toluene diisocyanate, adipic acid chloride, o-tolyl isocyanate, acetyl chloride, and acetic acid anhydride. The initiator system is reportedly useful in providing fast curing resins for dental applications.
One disadvantage of some of the aforementioned activators, however, is the relatively high odor of the activators and the composition in which they are used. Furthermore, another disadvantage of some of these activators is the relatively high level of mobile activator-amine reaction products (i.e., activator-amine constituents) in resulting adhesives. Typically, activators liberate the organoborane compound by bonding (either covalently or ionically) to the amine to form an activator-amine constituent. Most activator-amine constituents remain in the adhesive composition as mobile constituents that are not incorporated into the polymerized adhesive per se. In general, mobile constituents in adhesive compositions may cause performance problems when, for example, they migrate to the surface of the adhesive thereby disrupting the bond interface. Mobile constituents are also susceptible to attack by solvents, thereby making the adhesive composition less suitable for applications where exposure to solvents is unavoidable.
Recently, it has been disclosed that certain activators are useful for reducing the level of mobile constituents in adhesive compositions polymerized therewith. For example, PCT Publication No. WO97/07171 discloses bireactive decomplexers preferably comprising at least one free-radically polymerizable group and at least one amine-reactive group, preferably an isocyanate group, in the same molecule. The bireactive decomplexers are capable of forming a covalent bond with both acrylic monomers and the amine portion of the organoborane amine complex. Accordingly, the decomplexer is capable of covalently bonding to the liberated amine in the system and also reacting into the adhesive per se.
PCT Publication No. WO97/17383 discloses decomplexers comprising at least one anhydride group. Preferred anhydrides have one of the following structures:
wherein R1 and R2 may independently be an aliphatic group; a cycloaliphatic group; or an aromatic group. R3 is a divalent organic radical that completes a cyclic structure with the anhydride group. It is further preferred that the decomplexer comprises at least one free radically polymerizable group that is capable of forming a covalent bond with an acrylic monomer, such that the decomplexer-amine reaction product is capable of forming covalent bonds with acrylic monomer(s) and can be incorporated into the polymerized adhesive per se.
Such previous systems, however, may experience instability when exposed to hydroxy-functional materials, such as water and 2-hydroxyethyl methacrylate. For example, such hydroxy-functional materials may react with the isocyanate and anhydride functional groups on the decomplexers, resulting in a reduced amount of decomplexer available for activating the initiator systems. Thus, a need exists for further initiator systems that provide improved adhesive compositions, while maintaining low levels of mobile constituents and improved solvent resistance. It is also desirable to provide initiator systems that have relatively low odor.